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<jats:p>Chemostratigraphy of lavas in extrusive sequences of modern and ancient forearc environments provides significant information on oceanic crust formation through progressive stages of melt evolution from subduction initiation magmatism (SIM) to arc infancy, and arc maturation-related magmatism. We present here new trace element and radiogenic Sr–Nd–Pb isotopic data from a Late Cretaceous ophiolite in the Outer Zagros Belt of southern Iran and discuss its magmatic development through SIM in a Neotethyan forearc setting. Initial volcanism produced the stratigraphically oldest boninitic (BON) lavas in the Haji-Abad ophiolite whose trace element ratios as well as Pb isotopic signatures indicate a refractory mantle source, which was contaminated by subducting slab-derived fluids. The BON lavas change structurally upwards into island-arc tholeiitic (IAT) lavas and their fractionated felsic derivatives that display isotopic evidence for the contribution of sediment-derived melts in their magmatic source(s). This melt evolution pattern of the Haji-Abad ophiolite marks a major difference from the progressive melt evolution trends reported from the Izu–Bonin–Mariana (IBM) forearc lava sequence, which starts at the bottom with forearc basalts (FAB) phasing up-section into BON and IAT affinity lavas. IAT lavas in the IBM are in turn overlain by calc-alkaline (CAL) lavas. Our geochemical modelling points to a probable existence of a less depleted, pre-subduction mantle whose partial melting probably generated primary magmas before the eruption of boninitic lavas, although the record of such magmas is missing in the Haji-Abad ophiolite. Our comparison of the chemostratigraphy of the Outer Zagros ophiolites with the documented record of subduction initiation generated volcanic rocks in the other Neotethyan ophiolites in the region and the IBM forearc setting strongly suggests that there is no single rule or template for SIM.</jats:p> <jats:p content-type="supplementary-material"> <jats:bold>Supplementary material:</jats:bold> Details of analytical procedures and supplementary figures and tables are available at <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" specific-use="dataset is-supplemented-by" xlink:href="https://doi.org/10.6084/m9.figshare.c.6837597">https://doi.org/10.6084/m9.figshare.c.6837597</jats:ext-link> </jats:p> <jats:p content-type="thematic-collection"> <jats:bold>Thematic collection:</jats:bold> This article is part of the Ophiolites, melanges and blueschists collection available at: <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists">https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists</jats:ext-link> </jats:p>

<jats:p>The Oman ophiolite is one of the largest and best exposed ophiolites in the world and has &gt;450 chromitite deposits. We report here a newly identified chromitite deposit in the Wadi Rajmi, Oman. This deposit occurs within a dunitic envelope, which is surrounded by harzburgite, and consists of both massive and disseminated chromitite types. The Rajmi peridotites represent depleted upper mantle rocks that underwent &gt;20% partial melting and experienced metasomatism by melts and fluids derived from a subducting slab. They demonstrate geochemical affinities similar to those of the Izu–Bonin–Mariana forearc peridotites, supporting their formation in a forearc environment. The Rajmi chromitites have low Cr# values and are classified as high-Al chromitites. They have geochemical compositions comparable with those of chromitites crystallized from mid-ocean ridge basalt (MORB)-type melts. However, the chromites in these high-Al chromitites contain various silicate inclusions (e.g. amphiboles and micas), indicating the hydrous and atypical MORB nature of their parental magmas. Combined with the mineralogical and geochemical characteristics of the country rocks, we posit that the parental melts of the Rajmi high-Al chromitites have a MORB-like affinity derived from partial melting of a nascent forearc mantle.</jats:p> <jats:p content-type="supplementary-material"> <jats:bold>Supplementary material:</jats:bold> The whole-rock and mineral data of the Rajmi harzburgites, dunites and chromitites are available at <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" specific-use="dataset is-supplemented-by" xlink:href="https://doi.org/10.6084/m9.figshare.c.6795689">https://doi.org/10.6084/m9.figshare.c.6795689</jats:ext-link> </jats:p> <jats:p content-type="thematic-collection"> <jats:bold>Thematic collection:</jats:bold> This article is part of the Ophiolites, melanges and blueschists collection available at: <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists">https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists</jats:ext-link> </jats:p>

<jats:p> Ophiolite exposures in NW Croatia have been attributed to the Western Vardar Ophiolitic Unit and interpreted as derived from the Meliata–Maliac–Vardar branch of the Neotethys Ocean. Blocks within the ophiolitic mélange on Mt Ivanščica were investigated to determine the petrological and geochemical characteristics of the effusive rocks and to carry out radiolarian dating of the associated pelagic sedimentary rocks. The analysed effusive basic rocks represent chemostratigraphically uniform subalkaline high-Ti massive tholeiitic basalts characterized by an enriched composition typical of incompatible element-enriched mid-ocean ridge basalt (E-MORB). These basalts are compatible with <jats:italic>c.</jats:italic> 9–11% of partial melting of an enriched mantle source transitional between primitive and depleted MORB-type mantle and were formed in the non-subduction geotectonic setting of E-MORB-type rocks. This reflects an initial succession of oceanic protocrust formation and the onset of ocean spreading. Radiolarians from the chert and shale successions associated with the basalts indicate a Late Anisian–Early Ladinian age of initial ocean floor spreading, which continued into the Langobardian. The obtained data are correlative with reported blocks interpreted as remnants of the Triassic Neotethys crust from the ophiolitic mélange of the Western Vardar Ophiolitic Unit and reaffirm a common origin from a single ocean basin located east of the Adria microplate. </jats:p> <jats:p content-type="thematic-collection"> <jats:bold>Thematic collection:</jats:bold> This article is part of the Ophiolites, melanges and blueschists collection available at: <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists">https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists</jats:ext-link> </jats:p>

<jats:p> Lacustrine organic-rich shales in the second member of the Jurassic Qiketai Formation (J <jats:sub>2</jats:sub> q <jats:sup>2</jats:sup> ) in the Shengbei Sub-sag, Turpan–Hami Basin, NW China exhibit strong heterogeneity due to frequent alternations of sedimentary environments. The distinct shale environments present in both upper and lower units of J <jats:sub>2</jats:sub> q <jats:sup>2</jats:sup> provide an ideal example for studying the enrichment mechanism of organic matter (OM) under a complex sedimentary background. In this study, petrological, mineralogical, major/trace element and isotopic analyses were used to reconstruct the palaeo-environment and reveal the mechanisms of OM enrichment. The results indicate that the J <jats:sub>2</jats:sub> q <jats:sup>2</jats:sup> shale was deposited in a lacustrine mixed carbonate–siliciclastic environment and the palaeo-environment indicators suggesting an oxic–dysoxic, mildly brackish water condition. Based on a comprehensive comparison of both members of J <jats:sub>2</jats:sub> q <jats:sup>2</jats:sup> , an OM enrichment model is established and the main controlling factor of the formation of the organic-rich shale is elucidated. Under the background of a warm-humid climate, the lower unit of J <jats:sub>2</jats:sub> q <jats:sup>2</jats:sup> was deposited in a deeper and more restricted water body with stronger chemical weathering, resulting in limited terrestrial input compared to that of the upper unit. Inorganic geochemical analysis indicates a higher primary productivity in the lower unit of J <jats:sub>2</jats:sub> q <jats:sup>2</jats:sup> with local fluctuations. High primary productivity and favourable preservation conditions domain the OM enrichment in the study area. </jats:p> <jats:p content-type="supplementary-material"> <jats:bold>Supplementary material:</jats:bold> Raw data of carbon and oxygen isotopic analysis, TOC, Rock-Eval pyrolysis, and major and trace element analysis are available at <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" specific-use="dataset is-supplemented-by" xlink:href="https://doi.org/10.6084/m9.figshare.c.6901480">https://doi.org/10.6084/m9.figshare.c.6901480</jats:ext-link> </jats:p>

<jats:p> The External Ligurian Units are regarded as representative of the ocean continent transition between the Ligure‒Piemontese oceanic basin and the hyperextended Adria continental margin. The remnants of this transition are preserved as slide blocks embedded in a sedimentary mélange. This mélange sedimented during the Santonian to Campanian in the rear of the accretionary wedge that developed during the east-dipping subduction of the oceanic lithosphere below Adria. The main characteristic of this mélange is the occurrence of slide blocks of subcontinental mantle, lower continental crust, granitoids, gabbro, basalt and sedimentary rocks in a sedimentary matrix. In this paper, we provide petrographic observations coupled with U‒Pb zircon ages and <jats:inline-formula> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>δ</mml:mi> </mml:math> </jats:inline-formula> <jats:sup>18</jats:sup> O zircon isotopes of the slide blocks of felsic granulites and granitoids. U‒Pb data indicate ages ranging from Neoproterozoic to Late Triassic. <jats:inline-formula> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>δ</mml:mi> </mml:math> </jats:inline-formula> <jats:sup>18</jats:sup> O isotopes help to define the source of the magma of granitoids and felsic granulites, which was the continental crust. Overall, the collected data indicate that the slide blocks from sedimentary mélanges can be regarded as true archives that are able to provide useful constraints about the geodynamic history of the source area of these deposits. </jats:p> <jats:p content-type="supplementary-material"> <jats:bold>Supplementary material:</jats:bold> The dataset related to the U–Pb and ∂ <jats:sup>18</jats:sup> O analysisis available at <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" specific-use="dataset is-supplemented-by" xlink:href="https://doi.org/10.6084/m9.figshare.c.6922535">https://doi.org/10.6084/m9.figshare.c.6922535</jats:ext-link> </jats:p> <jats:p content-type="thematic-collection"> <jats:bold>Thematic collection:</jats:bold> This article is part of the Ophiolites, melanges and blueschists collection available at: <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists">https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists</jats:ext-link> </jats:p>

<jats:p>Active submarine channel bases are marked by large erosional features, such as knickpoints and plunge pools. Their presence in ancient channel-fills has rarely been documented, meaning their importance in submarine channel morphodynamics requires investigation. Using seismic reflection data calibrated by wells from a buried submarine channel-fill, we document erosional features 100s m long and 10s m deep, here interpreted as knickpoints and a plunge pool, and provide a mechanistic process for their transfer into the stratigraphic record for the first time. Channel incision patterns are interpreted to record a transient uplift in an otherwise subsiding depocentre. Local structural complexities in the channel slope formed zones of preferential scouring. A switch to a depositional regime preserved the irregular channel base inhibiting their upstream migration and smoothing of the channel base. Their formation and preservation record responses to salt tectonics and provide a unique snapshot of the formative processes of an ancient submarine channel. The presence of these exceptional basal scours indicates that headward erosion processes did not operate rapidly, challenging the paradigm that knickpoint migration controls channel evolution. Our results show that the primary erosion of the main channel surface, and long-term channel evolution, are dominated by far more gradual processes.</jats:p>

<jats:p>In Serbia ophiolitic mélanges occur widespread below ophiolites. These ophiolites are interpreted to derive from different oceanic domains and are therefore attributed to different tectonic units. We revisited all existing data from matrix ages and blocks in the mélanges, studied the relictic sedimentological features and dated new sections and blocks from various ophiolitic mélanges. On base of these results we can distinguish three different ophiolitic mélanges: 1. Intra-oceanic ophiolitic mélanges (OM1); 2. Ophiolitic mélanges formed during ophiolite obduction with continental blocks (OM2), and 3. Ophiolitic mélanges with fluviatile transported sedimentary rocks or tectonically incorporated much younger blocks at the base (OM3). These three types of ophiolitic mélanges resemble the polyphase history of shortening and ophiolite emplacement on the wider Adria plate of Serbia. All ophiolitic mélanges contain the same Triassic component spectrum of oceanic sedimentary cover rocks and have similar matrix ages. It can be concluded that all different ophiolites/ophiolitic mélanges derive from the same Triassic-Jurassic oceanic domain, the Neotethys Ocean which western part obducted during Middle-Late Jurassic times on wider Adria.</jats:p> <jats:p content-type="thematic-collection"> <jats:bold>Thematic collection:</jats:bold> This article is part of the Ophiolites, melanges and blueschists collection available at: <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists">https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists</jats:ext-link> </jats:p>

<jats:p> Remnants of coeval Devonian oceanic and continental foreland rocks are preserved in the basement of the North Patagonian Andes. Our previous studies of igneous rocks have shown the primitive oceanic and continental igneous rocks are coeval, and belong to a marginal basin that opened and closed over 50 Myr. A structural study and four new U-Pb SHRIMP ages and zircon Hf-O determinations allow identification of three metamorphic episodes, the first one M <jats:sub>1</jats:sub> (D <jats:sub>1</jats:sub> -S <jats:sub>1</jats:sub> ) coeval with Andean type granite intrusion in the foreland (405–380 Ma). This activity was simultaneous with development of an oceanic ridge and a marginal basin, at the outer edge of which a primitive granitic oceanic arc formed (380–385 Ma; zircon δ <jats:sup>18</jats:sup> O 3.6–5.2 δ <jats:sup>18</jats:sup> ‰). Ridge extinction initiated under-thrusting of the oceanic crust below the continent and an important mid-to-high grade metamorphic event M <jats:sub>2</jats:sub> (D <jats:sub>2</jats:sub> -S <jats:sub>2;</jats:sub> 375–360 Ma), its peak dated by metamorphic zircon rims in migmatite at 365 ± 3 Ma. Basin closure occurred after intrusion of S-type granites (357 ± 2 Ma; zircon δ <jats:sup>18</jats:sup> O 7.4–10.4‰) in the foreland, and accretion of gabbros, cumulate gabbros and trondhjemites at the proto-Pacific margin. Compression prevailed for 20 Myr in the foreland, causing a mylonitic medium-grade M <jats:sub>3</jats:sub> (D <jats:sub>3</jats:sub> -S <jats:sub>3</jats:sub> ) event. </jats:p> <jats:p content-type="supplementary-material"> <jats:bold>Supplementary material:</jats:bold> <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" specific-use="dataset is-supplemented-by" xlink:href="https://doi.org/10.6084/m9.figshare.c.6989873">https://doi.org/10.6084/m9.figshare.c.6989873</jats:ext-link> </jats:p>

<jats:p>We have investigated the geology, geochemistry, and geochronology of various plagiogranite intrusions that are spatially associated with ophiolitic subunits in a mélange terrain exposed within the Nain–Baft fault zone along the western boundary of the Central–East Iranian Microcontinent (CEIM). One group of plagiogranites is intrusive into foliated amphibolites, whereas the plagiogranites of the other group are intrusive into gabbros, dike swarms, and pillow lavas that are tectonically juxtaposed. U–Pb zircon dating of the first group of plagiogranites has revealed near concordant crystallization ages of 176.94±0.71, 179.84±0.92, and 190.40±2.1 Ma, indicating the early Jurassic timing of their emplacement. U–Pb zircon dating of the second group of plagiogranites has revealed a weighted average age of 92.34±0.38 Ma, indicating that they were emplaced in the early Late Cretaceous. The Early Jurassic plagiogranites are low–K, sub–alkaline rocks with negative Eu anomalies, low Ti and high La and Ce contents. They display elevated LILE/HFSE ratios. They were part of a magmatic system which facilitated the earliest rift–drift stages of a Neotethyan seaway development, and their magmas were produced by fractional crystallization of a mafic melt associated with asthenospheric upwelling beneath a rift system. The Late Cretaceous plagiogranites are low–K, sub–alkaline rocks with negative Eu anomalies, high Y and low Ta contents. They show LREE depletion relative to flat HREE patterns, and high Ba/Th ratios. Magmas of these plagiogranites were the products of partial melting of amphibolites in the subducting oceanic lithosphere in the same Neotethyan seaway. Thus, we posit that plagiogranite rocks in suture zones can be excellent trackers of geochemical, geochronological, and magmatic evolution of various types of oceanic crust through rift–drift, seafloor spreading, and subduction initiation stages within the same ocean basins in the past.</jats:p> <jats:p content-type="thematic-collection"> <jats:bold>Thematic collection:</jats:bold> This article is part of the Ophiolites, melanges and blueschists collection available at: <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists">https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists</jats:ext-link> </jats:p>

<jats:p>A review of the synorogenic basins formed on the Gondwana side of the Variscan orogen of Iberia is presented, highlighting the widespread ocurrence of calciturbiditic formations and olistostromes containing reef-limestone olistoliths in Iberia's Variscan basins. Using key-examples from the Variscan orogen for comparison (Azrou-Khenifra and Rhenohercynian basins), the significance of these olistostromes and flyschoid deposits is discussed. Our tectonic models of the Variscan belt in Iberia propose possible drivers of synorogenic carbonate platform/reef destruction responsible for the origin of calciturbidites and olistostromes. One model proposes the formation of an orogenic plateau by lateral flow of partially molten orogenic roots in the context of Laurussia-Gondwana convergence, following the destruction of the Rheic Ocean and Gondwana (lower) plate slab retreat. An alternative model invokes subduction of Paleotethys oceanic lithosphere beneath the Gondwana (upper) plate. Both emphasize the Mississippian occurrence of a significant thermal anomaly beneath Gondwana that favored strong lithosphere thinning, creating ideal conditions for synorogenic carbonate platform/reef destruction, and for the formation of calciturbidite deposits and olistostromes in Iberia. Variscan paleotopography would look alike in both situations. Thus, distinguishing these models is not straightforward, with differences in the kinematics of the regional tectonic transport in the superstructure of Mississippian gneiss domes.</jats:p> <jats:p content-type="thematic-collection"> <jats:bold>Thematic collection:</jats:bold> This article is part of the Ophiolites, melanges and blueschists collection available at: <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists">https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists</jats:ext-link> </jats:p>